In drilling applications today, a drill bit attached to the end of an extendable drill string can drill thousands of meters into the earth. There are several designs of mechanisms used today in drilling applications that allow the inner part of a drill string to be withdrawn from the hole using a wire and overshot, while the outer drill string stays in situ, thereby eliminating the need to withdraw the outer drill string each time a sample is taken. The method is commonly known as wire line drilling and is used as a method to retrieve rock samples, allow hole surveys or in some instances to change the type of drill bit. Using this method it is possible to drill downwards, upwards or horizontally.
Once the sample has been extracted the inner tube assembly is lowered/dropped or pumped back into the hole until it seats against a landing site, which may be a locating shoulder in the outer drill string. This locates the inner tube assembly in one direction axially. To locate the inner tube assembly in the other direction a set of latches may expand out from the inner tube assembly and shoulder a part of the outer drill string to prevent movement in the other direction. During the drilling process these latches are under load as the sample is pushed into the inner tube assembly.
When the inner tube assembly needs to be withdrawn an overshot member is lowered/pumped from the surface and locks itself to a barbed gripper or so called spearhead connected to the inner tube assembly. As load is taken by a wire line cable, the latches may be mechanically withdrawn, which releases the inner tube assembly and thereby allows it to be withdrawn from the outer drill string.
As stated above during drilling these latches take considerable load and under some drilling conditions they are forced tightly against e.g. a seating shoulder in the outer drill string and hence require a lot of force to retract them.
CA 2 223 511 discloses a core barrel apparatus with a wire line core barrel inner tube, having a main body portion of a latch body and an inner portion threaded together, where the latches are seatable in a drill string latch seat. The latches are connected via toggle links to a retractor pin, which in turn is retained within opposed apertures in a latch retractor tube. When the inner tube needs to be retracted, e.g. for retrieving core samples, an overshot member engages a spearhead, which is connected to the latch retractor tube. By an initial retraction of the overshot member, the retractor pin is moved outwardly without exerting radial outward forces through link pins, which are connected to the toggle links. Further retraction of the overshot member causes the retraction pin to move relative to link slots so that the link pins are moved outwardly and radially to pivot the latches out of the latch seat. Even further retraction of the retraction tube retracts the latch body completely.
A great initial axial force is required to release and retract the latches in the above disclosed core barrel apparatus.
U.S. Pat. No. 5,954,146 discloses a latch arrangement in a drill bit system, where a pair of latch dogs pivot about a pivot pin. A spring connects opposite ends of the latch dogs to a pin, which is able to move in a slot. When retrieving the latch dogs the pin is retracted and the spring retracts the ends of the latches.
Also with this arrangement the initial force required to release and retract the latches is great.
GB 1478127 discloses a locking mechanism, wherein the latches present a pair of substantially straight longitudinal edges, and present a cam groove, a withdrawal surface of which is straight and parallel with the longitudinal edges.
U.S. Pat. No. 3,667,558 discloses a locking mechanism, wherein the latches present a pair of longitudinal edges, and wherein a cam groove having a withdrawal surface with a ridge which will counteract a return movement of the cam follower.
There is hence a need for a latch arrangement that improves the initial leverage, i.e. reduces the forces taken up by the latches during retraction of the inner tube assembly.